1. Field of the Invention
The present invention relates to a variable magnification optical system and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system mountable in a video camera, an electronic still camera, and the like, and which is especially appropriate for use in a monitor or surveillance camera. Further, the present invention relates to an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, monitor cameras (surveillance cameras) were used to prevent crimes, to record images, and the like. Optical systems for the monitor cameras need to be small and producible at low cost. Further, the optical systems need to have large relative aperture, as well as high optical performance, so that subjects are identifiable even if photography is performed in low-illumination conditions. As a demand for monitor cameras with variable magnification functions increased in recent years, variable magnification optical systems are becoming the main trend of optical systems for monitor cameras.
Conventionally, zoom lenses for cameras were disclosed, for example, in Japanese Patent No. 2901066 (Patent Document 1) and U.S. Pat. No. 5,054,897 (Patent Document 2). Patent Document 1 discloses a zoom lens composed of a positive first lens group and a negative second lens group, which are arranged in this order from the object side of the zoom lens. Further, at least three of five lenses constituting the entire system of the zoom lens are made of plastic. Further, Patent Document 2 discloses a zoom lens composed of a negative first lens group and a positive second lens group, which are arranged in this order from the object side of the zoom lens. Further, a positive plastic lens and a negative plastic lens are arranged in the second lens group, or in each of the first lens group and the second lens group.
Conventionally, variable magnification optical systems for monitor cameras were proposed, for example, in Japanese Unexamined Patent Publication No. 2006-119574 (Patent Document 3), Japanese Unexamined Patent Publication No. 2008-310035 (Patent Document 4), and Japanese Unexamined Patent Publication No. 2008-310133 (Patent Document 5). Patent Document 3 discloses a variable magnification optical system composed of a negative first lens group, a stop, and a positive second lens group, which are arranged in this order from the object side of the variable magnification optical system. The magnification of the variable magnification optical system is changeable from a wide angle end to a telephoto end by moving the second lens group, on the optical axis, toward the object side. Further, a most-object-side lens in the second lens group is an aspheric lens. Further, Patent Document 4 discloses a variable magnification optical system composed of a negative first lens group, a stop and a positive second lens group, which are arranged in this order from the object side of the variable magnification optical system. When the magnification of the variable magnification optical system is changed from a wide angle end to a telephoto end, the first lens group and the second lens group move in such a manner that a distance therebetween decreases. Further, a most-image-side lens in the second lens group is a plastic lens. Further, Patent Document 5 discloses a variable magnification optical system composed of a negative first lens group, a stop, and a positive second lens group, which are arranged in this order from the object side of the variable magnification optical system. When the magnification of the variable magnification optical system is changed from a wide angle end to a telephoto end, the first lens group and the second lens group move in such a manner that a distance therebetween decreases. Further, at least one positive plastic lens and at least one negative plastic lens are arranged in the second lens group.
Currently, main trends of lenses for monitor cameras are high-performance lenses with wide angles and large relative apertures. Further, reduction in cost was strongly requested in recent years. One of methods for reducing the cost is use of plastic lenses. Meanwhile, the optical systems disclosed in Patent Documents 1 and 2 use many plastic lenses. However, they have problems, such as large F-numbers and small angles of view. In the optical system disclosed in Patent Document 3, all lenses are glass lenses. Especially because an aspheric lens made of glass is used, the cost for producing the optical system is high. The optical systems disclosed in Patent Documents 4 and 5 use plastic lenses, and the plastic lenses are arranged on the most image side of the optical systems. When rays of light pass through a most-image-side lens, the diameter of the rays of light is relatively small. Since dust tends to adhere to a plastic lens by static electricity more than to a glass lens, there is a problem that such adhesion of dust onto a surface of the plastic lens on the optical path of the optical system largely affects an image formed by the optical system.
Meanwhile, the advantages of a plastic lens over a glass lens are lower cost, lighter weight, and higher flexibility in shape (highly flexible designing). However, the plastic lens has disadvantages that the refractive index and the volume of the plastic lens greatly change by a fluctuation of temperature. Therefore, adoption of the plastic lens in a lens for an outdoor surveillance camera has been regarded as difficult, because a fluctuation of temperature is large outdoors.